(a) Technical Field
The present invention relates to a control method of a fuel cell system, more particularly to an operation pressure control method of a fuel cell system which appropriately maintains a differential pressure between hydrogen and air of a fuel cell stack to minimize a crossover amount of hydrogen, and reduces a hydrogen discharge amount at the time of purging hydrogen using a differential pressure between an anode outlet and a cathode outlet to improve a hydrogen utilization rate and system efficiency.
(b) Description of the Related Art
A fuel cell is an energy converting device which electrochemically reacts chemical energy of a fuel to be converted into electric energy without burning the chemical energy to be converted into heat, and which may be used not only to supply power for an industrial purpose, a domestic purpose, and a vehicle, but also to supply power of a small-size electric/electronic product or mobile equipment.
Conventionally, as a fuel cell for a vehicle, a polymer electrolyte membrane fuel cell (PEMFC) having a high power density has been utilized.
In the PEMFC, a membrane-electrode assembly (MEA) is located in an innermost part, and the MEA is configured by a solid polymer electrolyte membrane that may move a hydrogen ion and a cathode and an anode, which are electrode layers applied with a catalyst, on both surfaces of the electrolyte membrane so that hydrogen and oxygen react therewith.
On an outer part of the MEA, that is, on an outer part where the cathode and the anode are located, a gas diffusion layer (GDL) is laminated and a bipolar plate which supplies a reactant gas (hydrogen which is fuel gas and oxygen or air which is an oxidizer gas) and has a flow field through which a coolant passes is located on an outer part of the GDL.
A gasket for sealing fluid is laminated to be interposed between bipolar plates, after laminating a plurality of cells with the configuration as a unit cell, end plates are coupled to the outermost parts to support the cells, and the cells are arranged to be engaged with each other between the end plates to form a fuel cell stack.
Since each unit cell maintains low voltage at the time of operation, in order to increase voltage, several tens to several hundreds of cells are laminated in series to form a stack and then be used as an electricity generating device.
A fuel cell system which is applied to a fuel cell vehicle is configured by the above-described fuel cell stack and devices for supplying a reactant gas. FIG. 1 illustrates a diagram illustrating a fuel cell system.
As shown in FIG. 1, the fuel cell system includes a fuel cell stack 10 which generates electric energy from an electric chemical reaction of a reactant gas, a hydrogen supply device 20 which supplies hydrogen, which is a fuel, to the fuel cell stack 10, an air supply device 30 which supplies air containing oxygen to the fuel cell stack 10, a heat and water management system 40 which controls an operating temperature of the fuel cell stack 10 and manages water, and a fuel cell system controller (not illustrated) which controls an overall operation of the fuel cell system.
The hydrogen supply device 20 of a normal fuel cell system includes a hydrogen storage unit (hydrogen tank, not illustrated), a regulator (not illustrated), a hydrogen pressure adjusting valve 21, and a hydrogen recycling device 22, the air supply device 30 includes an air blower (atmospheric pressure type) or an air compressor (pressurized type) 32 and a humidifier 33, and the heat and water management system includes a water trap 41, an electro-motion water pump (a coolant pump), a water tank, and a radiator which are not illustrated.
A high pressure of hydrogen which is supplied from the hydrogen supply device 20 to the hydrogen tank is reduced to a predetermined pressure in the regulator and then the hydrogen is supplied to the fuel cell stack 10. In this case, the hydrogen having a reduced pressure is supplied to the fuel cell stack with a controlled supply amount through pressure control in accordance with an operating condition of the fuel cell stack.
That is, the pressure of the hydrogen which goes through the regulator from the hydrogen tank is adjusted by the hydrogen pressure adjusting valve 21 at an inlet of the anode of the stack, and then the hydrogen is supplied to the fuel cell stack 10. The hydrogen pressure adjusting valve 21 is controlled to adjust the pressure of the hydrogen which is reduced by the regulator to be an appropriate pressure in accordance with the stack operating condition. In this case, the controller receives values of two hydrogen pressure sensors 25 and 26 which are provided at the inlet and outlet of the anode of the stack to control the hydrogen pressure adjusting valve 21.
The hydrogen which remains in the fuel cell stack 10 after reaction is discharged through the outlet of the anode (anode) of the stack or recycled to the inlet of the anode of the stack by the hydrogen recycling device 22.
The hydrogen recycling device 22 is a device which increases reliability of supplying hydrogen and improves a life span of the fuel cell. There are several recycling methods and a method using an ejector 23, a method using a blower, and a method using both the ejector and the blower are known.
The hydrogen recycling device 22 recycles unreacted hydrogen which remains after being used in the anode (anode) of the fuel cell stack 10 back to the anode through a recycling pipe 24, thereby trying to reuse the hydrogen.
In the fuel cell, as foreign substances such as nitrogen, water, and moisture which is transferred to the anode through an electrode membrane in the stack are increased, the amount of hydrogen in the anode is reduced, so that reaction efficiency is lowered and thus the purge valve 27 is opened in accordance with a predetermined cycle to purge the hydrogen.
That is, the purge valve 27 for purging hydrogen is provided at the outlet side pipe of the anode of the fuel cell stack 10 to periodically discharge the hydrogen in the anode, to discharge and remove the foreign substance such as nitrogen or water from the fuel cell stack and increase a hydrogen utilization rate.
As described above, when the foreign substance in the fuel cell stack is discharged, there are advantages of an increased hydrogen concentration, an increased hydrogen utilization rate, and improved gas diffusion degree and reactivity.
A method of operating a fuel cell system is mainly divided into an atmospheric pressure type and a pressurized type, and an operation pressure of the fuel cell stack in each operating method acts as one of the factors that affects performance.
The atmospheric pressure type fuel cell system generally uses an air blower to supply atmospheric pressure air to the cathode of the stack, and the pressurized type fuel cell system uses an air compressor 32 to supply air having a higher pressure than the atmospheric pressure to the cathode of the stack.
The pressurized type fuel cell system supplies air which passes through a filter 31 to the cathode of the fuel cell stack 10 using the air compressor 32 and controls a pressure of the outlet of the cathode using the air pressure adjusting valve 34 which is mounted in the outlet side pipe of the cathode of the stack.
In the meantime, the hydrogen purge which is periodically performed on the anode using the purge valve 27 is performed using a differential pressure between the hydrogen side and the air side in order to improve hydrogen concentration in the fuel cell system.
In this case, as the differential pressure is increased, an amount of hydrogen which is crossed-over from the anode to the cathode through the MEA in the fuel cell stack in the normal driving section is increased, which lowers the utilization rate of the hydrogen.
Therefore, an operating technique which minimizes a hydrogen cross-over amount while allowing the hydrogen purge by appropriately maintaining the differential pressure of the hydrogen side and the air side is required.
In the related art, the operation pressure at the hydrogen side is normally maintained to be higher than an operation pressure at the air side and in this case, hydrogen purge using an operation pressure difference between the anode outlet and the cathode outlet is allowed, which is advantageous to secure a stable system operation.
However, when the operation pressure is increased as illustrated in FIG. 2 (RELATED ART), due to the increase of air flow, the differential pressure between the anode outlet and the cathode outlet is increased, so that one purge flow of hydrogen when the purge valve is opened is gradually increased and the hydrogen operation pressure is raised, thereby gradually increasing an amount of hydrogen which is crossed-over from the anode to the cathode.
However, the increased hydrogen purge amount and the increased hydrogen cross-over amount may be major causes of lowering the hydrogen utilization rate and system efficiency.
Specifically, in the case of a pressurized operating system, the differential pressure between the anode outlet and the cathode outlet is further increased in accordance with increase of an operation pressure of a cathode inlet. Further, when the differential pressure between the cathode inlet and the cathode outlet is increased due to the system design change (for example, design change of a bipolar plate), the differential pressure between the anode outlet and the cathode outlet is further increased.
FIG. 2 illustrates an operation pressure control map of the related art in accordance with an air mass flow in which A (kPa) indicates a minimum operation pressure of an anode inlet and an anode outlet.
Generally, when a target air mass flow is determined in accordance with the fuel cell operating condition at the time of controlling the operation pressure of the fuel cell system, a controller determines target values of pressures of the anode inlet and the anode outlet and the cathode inlet and the cathode outlet corresponding to the target flow from the operation pressure control map and receives the measurement values of the hydrogen pressure sensors 25 and 26 and the air pressure sensors 35 and 36 to control the pressures of the anode inlet and outlet and the cathode inlet and the cathode outlet to be the target pressure values.
Here, the operation pressure of the fuel cell system is set in the map to be controlled such that the cathode inlet pressure is higher than the cathode outlet pressure, the hydrogen outlet pressure is higher than the cathode inlet pressure, and the anode inlet pressure is higher than the anode outlet pressure, as illustrated in FIG. 2.
Here, the excessive hydrogen operation pressure increases a hydrogen cross-over amount, which lowers the hydrogen utilization rate and the system efficiency.
When the differential pressure is excessively high to discharge a gas of the anode using a differential pressure between the anode outlet and the cathode outlet at the time of hydrogen purge, the discharge amount when the purge valve is opened one time is excessively large.
Therefore, it is required to improve an operating speed of the purge valve in order to reduce the discharge amount when the purge valve is opened one time, but this may cause the development cost to be increased.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.